Forward osmosis technology is known to be used as a method for producing purified water (Patent Documents 1 and 2).
Forward osmosis technology is a technology whereby, after contacting raw water to be purified with an induction solution containing water and high concentration of separable solute through a semipermeable membrane, and extracting only the water in the raw water into the induction solution, purified water is obtained by removing the solute from the induction solution. A water purification system that uses forward osmosis technology does not require the creation of an artificial pressure difference since extraction of water from the raw water into the induction solution is driven by an osmotic pressure difference.
In general, composite membranes used as positive osmosis membranes are manufactured by forming an active layer composed of a thin film on the surface of a supporting membrane. The formation of this active layer is carried out by, for example, a coating method, interfacial polymerization or plasma polymerization.
Interfacial polymerization is a technology consisting of respectively dissolving two types of reactive monomers in water and an organic solvent not miscible with water, and forming a polymer by enabling the monomers to react at the interface of the two solutions by allowing the solutions to make contact. A composite membrane capable of being used as a forward osmosis membrane can be obtained by carrying out this interfacial polymerization reaction on the surface of a microporous supporting membrane. The manufacturing of a composite membrane by a commonly known interfacial polymerization method is carried out in the manner indicated below using two types of reactive compounds capable of forming a polymer through the mutual reaction thereof.
Namely, a first solution containing one of the reactive compounds and a second solution containing the other reactive compound while also being immiscible with the first solution are prepared. A microporous supporting membrane is then immersed in the first solution followed by immersing in the second solution after having removed any excess first solution. As a result, interfacial polymerization of the reactive compounds is carried out on the surface of the microporous supporting membrane. A composite membrane having a thin film on the surface of a microporous supporting membrane is then formed by removing the solvent of the second solution.
Methods for forming a polymer thin film on the outer surface of a microporous supporting membrane in the form of hollow fibers by interfacial polymerization are well known. For example, a method is known whereby a guide roll is provided in a reaction solution tank and a microporous hollow fiber supporting membrane is continuously immersed in the reaction solution by passing through this guide roll (Patent Documents 3 and 4).
Technology for forming a polymer film on the outer surface of hollow fibers offers the advantage of being able to be carried out continuously following a spinning step. However, this technology has the problem of damaging the formed polymer film due to contact with the guide roll and contact between hollow fibers when filling into a module.
In contrast, in the case of forming a polymer thin film on the inner surface of hollow fibers, the polymer film can be formed after having integrated the hollow fibers into a module, thereby preventing damage to the polymer film during subsequent handling.
A known example of a method for forming a polymer thin film on the inner surface of hollow fibers consists of forming a liquid film of a first solution on the inner surface of the hollow fibers by filling the first solution into the hollow portions of the hollow fibers and then removing any excess solution using high-pressure air, followed by passing a second solution through the hollow portions of the hollow fibers (Patent Document 5).
Another known method consists of forming a polymer thin film on the inner surface of hollow fibers by coating a prepolymer or oligomer onto the inner surface of the hollow fibers followed by post-crosslinking (Patent Document 6).